The invention relates to the field of gun-launched guidance systems and to a navigation system based on inertial sensors mounted in a spinning projectile using at least one rotation sensing device with input components perpendicular to the spinning body's longitudinal axis, or at least one acceleration sensing device with input components along the spinning body's longitudinal axis.
A projectile in flight follows a trajectory defined by an interaction of gravity, aerodynamics, and mechanical forces due to spin, shape and possible steering fins. The projectile's flight phases can be described in terms of a pre-launch phase, launch phase, and ballistic phase.
In the pre-launch phase, before launch of the projectile (e.g., an artillery shell), enough navigation information is available to perform a pre-launch alignment of the on-board inertial navigation system. The launch phase is characterized by high-G forces that occur during launch. During the launch phase, most navigation systems will not be able to navigate due to these high-G forces, and it is necessary to perform a post-launch alignment of the inertial system, as described below.
After the launch phase, i.e., at the start of the ballistic phase, the navigation system has to be aligned before it can navigate. At launch, parameters such as elevation angle, muzzle velocity, heading and spin rate are known to an extent needed for a coarse alignment of the navigation system. The roll angle (the angle about the projectile's longitudinal axis, or axis roughly in parallel with its direction of travel) of the projectile is however not known. Due to the projectile's spin, the roll angle is also rapidly changing. The roll angle must therefore be established to a degree that the coarse alignment accuracy provides a sufficient initialization for a successful subsequent fine alignment phase. This process of estimating the roll angle in a spinning projectile is referred to as ‘Upfinding’.
During the ballistic phase of the trajectory, the pitch angle of the shell will decrease at a small angular rate. When the shell spins, the pitch rate can be observed in an axis perpendicular to the spin axis as a sinusoidal rate, where the maximum and minimums occur when that axis is in the horizontal plane, see FIG. 1. The phase of the sinusoidal rate in an axis perpendicular to the projectile's spin axis can therefore be used to indicate the shell's roll angle. An accelerometer with its input axis co-aligned with the shell's spin axis and mounted off center in the shell will pick up a sinusoidal Coriolis acceleration due to the interaction of its velocity vector around the shell's center and the change in the shell's pitch rate. The phase of the sinusoidal Coriolis acceleration can also be used to indicate the shell's roll angle.
Inertial sensors that are used to determine positional and orientation parameters, including the time derivatives of these parameters, generally exceed their operational ranges during the high-g shock at launch.
In an existing solution to the upfinding problem, a pitch- (or yaw-angle gyroscope is positioned in the shell to detect rotation in an axis perpendicular to the spin axis. The pitch-angle gyroscope detects the change in the shell's pitch angle as the shell travels in a ballistic trajectory. As the shell spins around its longitudinal axis, the gyroscope in the perpendicular axis picks up the shell's pitch rate as a sine wave. The phase of this sine wave is directly related to the shell's roll angle and can be used to estimate the roll angle. In particular, as the input axis of the gyroscope points upward or downward, the detected rotation is approximately zero; when the axis of the gyroscope is horizontal, the gyroscope senses maximum positive or negative pitch rate.
Alternately, an accelerometer with its input axis along the shell's longitudinal axis can use the Coriolis acceleration to estimate the shell's roll angle. The measured Coriolis acceleration will also exhibit a sine wave related to the shell's roll angle.
The existing solutions to the upfinding problem are described, e.g., in Lucia, D. J., “Estimation of the Local Vertical State for a Guided Munition Shell with an Embedded GPS/Micro-Mechanical Inertial Navigation System”, MIT Masters of Science Thesis, May 1995 (“Lucia”), and Gustafson, D. E., Lucia, D. J. “Autonomous Local Vertical Determination for Guided Artillery Shells”, Autonomous Local Vertical Determination for Guided Artillery Shells, D. Gustafson, Draper Laboratory; D. Lucia, Falcon AFB, pp213-221 52nd Annual Meeting Proceedings “Navigational Technology for the Third Millennium” Jun. 19-21, 1996, Royal Sonesta Hotel, Cambridge, Mass. (“Gustafson & Lucia”).
U.S. Pat. No. 5,886,257 describes an apparatus and a method for making an autonomous local vertical determination for a ballistic body using recursive Kalman filtering to determine the roll angle (local vertical direction).
U.S. Pat. No. 5,372,334 describes the use of a retroreflector mounted on the projectile to implement an improved local vertical reference determination.
U.S. Pat. No. 6,163,021 describes a navigation system for spinning projectiles utilizing a magnetic spin sensor and a GPS/INS Kalman filter.
An article by Bar-Itzack, I. Y., Reiner, J. and Naroditsky, M., titled “New Inertial Azimuth Finder Apparatus”, AIAA Journal of Guidance, Control and Dynamics, Vol. 24, No 2, March-April 2001, pp 206-213 cites Israeli Patent 129654, filed Apr. 28, 1999, titled “Method and Apparatus for Determining the Geographical Heading of a Body,” that discusses finding a geographical north of a body.
While these references disclose various ways of finding a solution to the upfinding problem, none of them disclose an optimized system utilizing a phase-locked-loop (PLL) or a correlator or the enhancement from complementary filtering the roll angle with roll rate.